Monocytes are an important leukocyte subset for potential clinical use as cell therapy agents. In culture, and with the addition of the appropriate cytokines, monocytes can differentiate into other clinically useful cells such as activated monocytes/macrophages (1-10), dendritic cells (11-15), endothelial-like cells (16-21), osteoclasts (22-23), and microglial cells (24). For example, monocytes/macrophages have been utilized for the experimental regeneration of central nerve tissue, the clinical treatment of chronic wounds, and adoptive immunotherapy. In animal models, monocytes/macrophages pretreated by exposure to damaged, regeneration-competent peripheral nerve imparts to macrophages the ability to promote the experimental regeneration of transected optic nerve (1) and spinal cord (2). Phytohemagglutinin stimulated allogenic mononuclear cells have been locally applied to venous or arterial ulcers in patients, resulting in accelerated granulation, epithelialization, and a greater likelihood of complete closure compared to untreated controls (3). Subcutaneous administration of allogeneic activated adherent monocytes/macrophages into the healthy tissue surrounding wounds promoted wound repair and closure in a murine system (4) and in patients with decubitus ulcers (5,6). Monocytes have also been cultured in autologous serum and recombinant granulocyte/macrophage-colony stimulating factor to promote differentiation 5 into macrophages, which have been subsequently activated with interferon gamma and other agents to produce macrophage-activated killer (MAK) cells for the treatment of cancer (7-10). Monocyte derived dendritic cells are being extensively investigated as professional antigen presenting cells and are being used in the laboratory in cellular-based vaccines for infectious diseases and are in use clinically for the treatment of cancer (11-15).
Non-Filter Methods for Monocyte Isolation:
There are several methods for the isolation of monocytes for clinical use. Large numbers (>1×109) of highly pure monocytes (>90%) can be isolated by density gradient sedimentation followed by counterflow centrifugal elutriation of mononuclear cells collected by apheresis (25). Those cells are somewhat activated immediately following isolation, but become quiescent after overnight incubation in a defined media (26). However, no closed system elutriator has yet been developed for sterile monocyte isolation. Alternatively, large numbers of monocytes (>1×109) can be purified by performing a continuous density gradient sedimentation to isolate mononuclear cells followed by either a continuous or discontinuous density gradient sedimentation to separate monocytes from lymphocytes (27,28). However, the technique requires two gradients, discontinuous gradients can be difficult to prepare, and the procedure is not easily carried out in closed systems. Monocytes have also been isolated by permitting attachment of cells derived from osmotically shocked buffy coats to the polyvinyl chloride polymer and plasticizers of blood containers (6). Cells isolated by adherence to blood container plastic are activated with or without osmotic shock, and have been shown to secrete pro-inflammatory cytokines during culture (29,30). While this technique can be performed in a closed system, it typically produces relatively small numbers of cells (<5×107) compared to elutriation methods. In addition, no method for cryopreservation of adherent monocytes from osmotically shocked buffy coat has been reported. Another disadvantage is that non-filter methods are time consuming and often require more than two hours to complete the isolation procedures. Therefore, there is a clinical need for the development of methods for sterile and rapid isolation of adherent monocyte preparations containing large numbers of cells that can be cryopreserved. Such preparations would facilitate the logistics of conducting clinical trials, including dose-ranging studies. These clinical applications might include the use of monocytes/macrophages for the healing of chronic wounds, the administration of endothelial cells for healing of chronic wounds, the use of endothelial cells for supplying new vessels to support repair of damaged heart tissue, the use of dendritic cells for the preparation of infectious disease or cancer vaccines, the use of osteoclasts for the control of pathological conditions resulting in excess bone formation and atherosclerosis (31), and the use of microglial cells to remove damaged central nervous tissue.
Filter Methods for Monocyte Isolation:
One source of leukocytes from which monocytes might be derived is from non-woven filters used to deplete leukocytes from blood and blood components. For example, leukocytes, including neutrophils, eosinophils, lymphocytes and monocytes, were recovered from a leukocyte depletion filter by Chong and coworkers by backflushing with cold, anticoagulant sodium citrate solution (32). One disadvantage of such a method, however, is that many of these leukocyte reduction filters bind platelets, which are released upon backflushing the filter and thus contaminate the final product. Platelets can interfere with the further culture of monocytes to dendritic cells, osteoclasts, and endothelial cells.
Ebner and colleagues utilized blood leukocyte reduction filters as a source of leukocytes along with two additional non-filter purification processes to eventually culture monocytes into dendritic cells (33). In that study, the authors used density gradient centrifugation to remove neutrophils and T-cells rosetting with sheep erythrocytes to remove residual red cells as a means to isolate monocytes from a leukodepletion filter.
An advantage of using filters as a source of leukocytes is that large quantities of monocytes can be aseptically collected with little contamination by red cells, platelets, and plasma. Additionally, filtration methods afford especially high quantities of activated monocytes. Activated monocytes exhibit pluripotency and can be cultured and differentiated into numerous other lineages including dendritic cells, macrophages, osteoclasts, endothelial cells, and microglial cells. Activated monocytes also secrete greater quantities of cytokines associated with inflammation and differentiation. Such activated monocytes also display dendritic cell surface markers earlier during culture than monocytes collected by other methods.
Non-woven filters such as are used in cord blood filters or stem cell filters can be used to concentrate monocytes from blood component mixtures. These monocyte adhering filters preferentially bind white cells over plasma, platelets, and red cells; and preferentially bind monocytes over lymphocytes or granulocytes. Other examples of monocyte adhering filters include whole blood leukoreduction filters, which also bind white cells preferentially over plasma, platelets, and red cells. As used herein, the term monocyte-adhering filter refers to a filter that will pass about 90% of the red cells in a blood component mixture, and about 75% platelets, yet retains at least about 75 to 100% monocytes, about 20 to 80% lymphocytes, and 10 to 50% granulocytes. A blood component mixture is any mixture derived from blood or blood products that includes leukocytes, and in particular monocytes.
An example of a monocyte-adhering filter is a commercially available non-woven filter of superfine polyethylene terephthalate fibers coated with about 97% 2-hydroxyethyl methacrylate and about 3% N,N-dimethylaminoethyl methacrylate. (SCF System, Asahi Medical Corp.) (34). Such filters pass about 90% red cells and about 75% platelets, while retaining at least about 86% monocytes, about 74% lymphocytes, and about 31% granulocytes.
Using a backflush solution of Dextran-40 solution and serum albumin under high shear conditions, it is possible to recover greater than 80% of the CD34+ cells in a blood component mixture as well as about 80% of cells with longterm culture initiating cell (LTC-IC) activity. Stem cells and monocytes isolated from backflushing the filter were still contaminated with substantial numbers of lymphocytes and neutrophils; therefore, preparations still contained low percentages (10%) of monocyte subpopulations (34).
Existing filtration collection methods suffer several disadvantages. They require additional purification to adequately enrich monocytes for potential clinical use. Most, if not all, of the purification methods are difficult or impossible to carry out in a closed system, making maintenance of sterility a problem in producing clinical grade cell preparations. Similarly, most purification procedures require more than two hours for completion, making the isolation process costly and time consuming.
There is a need in the art for apparatus and methods that provide rapid means for differentially separating white cell subpopulations from the filter itself, and that avoid the need for additional separation and sterilization techniques. This would provide means for the sterile isolation of large numbers of monocytes in a closed system with adequate purity. The resulting cells could be used directly for cell therapy, or cultured in closed systems for use by those skilled in the art for differentiation into macrophages, dendritic cells, osteoclasts, endothelial-like cells, or microglial cells, and could subsequently be used in cellular therapeutic procedures.